Selective calling system



April 1951 D. E. NOBLE 2,547,024

SELECTIVE CALLING SYSTEM Filed July 2, 1947 a Sheets-Sheet 1 FIG. 1

IO E A I8 21 22 23 24 25 0 Audio c RF Deiecior Audio limp. l3 Tmnsmmer Amp. or Discr. Amp. Tone Modulmor I Seleciive Gen. 16 gg gr o '8iPoiverUnii. gg/ syst Tone OIIB 2 O 24 x Gen. P- 1 i 5 RF. Deiecigr Audio 1 188g (tEoMixer 7 i I Amp. or Discr. Amp. and l s[ l6 eeciive g /Coiling Sysi.

FIG 2 O u I 28 24 25 27 29 0 Audio Wire Line 2 Audio Amp. 5 Amp. T m

one 1 a Selective TE 2:? CoiiinqSsi. 26

i6 1 ggg Amp. 24\ 25 l5 Audio 82? Mixer d o I Tone on Seieciive 1 Gen. 7- Ooiiinq Svsi. 26

INVENTOR. Daniel E. Noble dle/KM A'riorney April 1951 D. E. NOBLE 7 2,547,024

SELECTIVE CALLING SYSTEM Filed July 2, 1947 3 Sheets-Sheet 2 Filter and Rectifier FIG.

Filter and Reotitler 174 FIG. 5

95 96 98 9 93 0 (I 0- -O (I c r H Detector Selective l Amplrfrer Responsive A Drcrrmrnotor 0 De ice Rectifier I 5 o INVENTOR.

Daniel E. Noble BY Attorney April 3, 1951 D. E. NOBLE SELECTIVE CALLING SYSTEM 3 Sheets- Sheet 5 FIG. 6

Filed July 2, 1947 elective Responsive Device I04 INVENTOR. Daniel E. Noble Patented Apr. 3, 1951 SELECTIVE CALLING. SYSTEM Daniel E. Noble, Elmhurst,. Ill.,. assignor to M- torola, Inc., Chicago Ilh, a. corporation of Illinois Application.July 2, 1947,. Serial No; 758,496

9 Claims.

This invention relates generally to systems for providing selective calling or selective switching in. radio or wire communication equipment and in particular to a system in which the calling signals and communication signals are transmitted over a single channel.

In. radio communication networks such as used by police, or in any other installations. including a central station and a plurality of remote stations, it is necessary that some signal system be provided so that an operator at the. central station can call an operator at aremote station or perform a selective switching operation on' remote equipment. Suchsystems may provide an audible or visual signal to notify the called 0perator, or preferably may be connected into the remote radio stations in such a way that the signal will automatically condition the receiver at a particular remote station. for operation. These systems may also be used to provide some other switching function such. as to turn on standby units.

Prior systems provided for this purpose have been patterned after selective calling equipment used in telephone systems with the result that the equipment has been relatively complicated and consequently expensive. Also in many radio communication systems as above described the remote stations are mobile. units and the equipment therefor must be rugged. in order to: provide dependable service when: subject to vibration and shock encountered in a vehicular installation. The prior selective calling equipment. being complicated is not well adapted to vehicular use and also requires a relatively large amount of space which is not generally available in such vehicular installations.

In order to provide. a selective calling system utilizing a minimum of equipment, the system should be such. that the calling signals can be transmitted over the. usual communication channel; Further, the system should. be. flexible so that it. is capable. of providing the number of different calls required in a more. complex system and still provide a relatively simplesystem for use where. a. relatively small number of.v calls is: required.

It is,. therefore, an object of: the present. invention to provide an improved selectivecalling system ior radio communication networks which is of: rugged construction;

It is another. object. of. this invention to provide a selective calling system for. a radio. communication. network in which the calling signals and:v the: voice. signals are; transmitted. over a single channeland. which discriminates between the calling signals. and the voice signals.

A furt her object of this invention is to provide a comparatively simple. and inexpensive calling system which: is particularly applicable tofrequency modulation radio. communication networks.

A feature of this invention is the provision. of a selective calling system which derives the energy required for operation thereof from. a tone of a particular. frequency.

A further. feature of this invention is the provision of a selective. calling system adapted. to be operated by a pair of calling signals one of which acts as. a carrier for the other signal.

A still further feature of this invention. is. the provision. of a selective. responsive device including. an electromechanical contactor responsive to a particular frequency and means for energizing said device from a tone of a different particular frequency so that the device responds to operate controls upon. simultaneous actuation. of. said contactor and application of. energy thereto.

Further objects, features. and advantages will be apparent from a. consideration of the following description taken in. connection with the accompanying'v drawings in which:

Fig. 1 is a block. diagram illustrating. a radio communication network utilizing the selective calling system in accordance with this invention;

Fig. 2. is a. similar network utilizing wire lines;

Fig. 3 is a circuit diagram of one embodiment of. the selective. calling system in accordance with the invention;

Fig. 4 is a circuit diagram of amodified system capable of handling. a large number of different calls;

Fig. 5 is a blockdiagram illustrating a selective calling system operable by two tones one of which serves as a. carrier for the other;

Fig. 6 is a. circuit diagram. of. a. frequency modulatedv system; and' Figs. 7 and 3- illustrate. various filter rectifier unitsadapted for use in the systems of Figs. 3, 4, 5 and 6.

In. practicing the. invention there is provided acommunication system comprising a. transmitting station in which voice or other communication. signals and. calling signals in. the audio range can be alternatively transmitted- A receiving station is provided in which. the. voice and calling, signals are detected and. which includes a selective calling system for conditioning a s0und reproducing device for reproducing voice signals in response to the transmission of calling signals. Such a system requires only a single channel and either radio or wire transmission may be used. The. selective calling system includes a signal. responsive device having an electromechanicalcontactor adapted to respond. to a signal of a particular frequency and means for supplying energy to the device when atone of a particular frequency is; received. Simultaneonsactuation. of. the contactor. and application 015 energy to -the-selective responsive device is required for actuation of the calling system. The energizing tone may be used as a carrier for the signal which actuates the contactor, the signal later being separated by a detector or discriminator depending upon the type of modulation.

including a microphone l0, audio amplifier II, and a transmitter modulator and power amplifier l2. A switch I3 is provided for selectively connecting one of a plurality of calling signal generators M and I5 to the transmitter. Each of the calling signal generators includes two or more tone generators l5 and a mixer and amplifier ll for combining the tones prior to application to the transmitter modulator and power amplifier l2. The mixer may be either a simple transformer for combining the signals from the tone generators or may be a modulator for modulating one of the tones with the other tones as will be described. An antenna 58 is provided for radiating the signals from the transmitter 52.

The communication network may include a plurality of receivers. two receivers l9 and being illustrated in Fig. 1., Each of these receivers includes an antenna 2!, radio frequency amplifier 22, detector or discriminator 23, audio amplifier 24 and sound reproducing device 25. A

selective calling system 26 is provided for selectively connecting the sound reproducing device to the audio amplifier. The various receivers in the network may be identical except that the selective calling system of each receiver will be arranged to respond to a particular calling signal as produced by one of the calling signal generators M or 15. The various items of equipment illustrated in Fig. 1 are well known and may be of any suitable construction, the invention being directed specifically to the selective calling .I system 26 shown in "this network.

The communication system utilizing the selective calling system in accordance with the invention in a wire system is illustrated in Fig. 2. The

1 system is generally similar to that of Fig. 1 and corresponding parts are designated by the same reference characters. A microphone it is connected through an audio amplifier H to a primary winding to transformer 2'1. Calling signal generators i4 and I5 may be selectively con- 'nected through switch l3 to a second primary winding of the transformer 21. The transformer 21 includes a secondary winding which is connected through wire line 28 to transformer 29. f The transformer 29 has a plurality of secondary windings connected to audio amplifiers 24 which are in turn connected to sound reproducing devices and selective calling systems 26. This network is merely illustrative and various other arrangements for use with wire lines can be used. Also the wire line need not be a simple 'two-wire line as shown but can be any wire L channel capable of carrying signals in the audio range.

In Fig. 3 there is illustrated the detailed ciricuit of a selective calling system in accordance with the invention which may be used in the communication systems illustrated in Figs. 1 and 2. The transformer 30 is adapted to be connected to any signal source which includes callin signals and also communication signals and may be, for example, the output transformer of an audio amplifier of a radio receiver as illustrated at 24 in Fig. l. The transformer 36 includes a primary winding 3! for connection to the signal source and three secondary windings 32, 33 and 3d. The secondary winding 32 is connected to a sound reproducing device 35 through the contacts 36 of a relay 3?, the secondary winding 33 is connected to the winding 38 of contactor 3a of the selective responsive device 46 and the winding 34 is connected to .a filter and rectifier unit 50. The signal is therefore independently applied to each of these components.

The selective responsive device 46 includes in addition to the contactor 39 and relay 3'5 a charging circuit energized by the unit 5G and an electron discharge valve 2 5 which controls the operation of the relay. The contactor may be any electromechanical device which will cause a circuit to be closed when a signal of a particular frequency is applied thereto. For example. vibrating reeds are commercially available having contacts thereon which establish momentary electrical connection with the stationary contact points during the course of vibration of the reed. Coils are provided to produce a magnetic field about the reed which fluctuates in accordance with the frequency of the signal applied thereto, the reed responding only when the signal applied to the coil is the same as the natural frequency of the reed. The electromechanical contactor 39 is bridged by condenser fit and connected in series with a resistor 4i, condenser 42 and terminals 43 which are adapted to have a voltage impressed thereacross. (A resistor 44 is connected in parallel to the condenser 42 serving to dissipate any energy stored in the condenser to thereby discharge the same. The contactor 38 will be normally open but will be intermittently closed when the proper frequency is applied to the winding 38. It is apparent that closing the contactor 39 causes the condenser 42 to be connected in series with resistor 4! across the voltage source supplied through terminals 43 to thereby charge the condenser. The intermittent closing of contactor 39 will cause increments of charge to be built upon condenser until the voltage across the condenser approaches that of the voltage source. This voltage is applied to the grid i l of electron discharge valve 45 which is illustrated as a triode. When the voltage on the grid 46 reaches a certain point the current in the output circuit of the tube 45 will increase in the normal manner until sufiicient current flows through the winding 3? to actuate the relay causing the contacts 35 thereof to close to connect the loud speaker.

As previously stated, the winding 34 is connected to a filter and rectifier unit 50 which is adapted to apply a voltage across the terminals 43 of the selective responsive device when a tone of a particularly frequency is received from the signal source. The filter and rectifier unit 50 can be of any suitable construction, examples of units which are satisfactory being illustrated in Figs. 7 and 8. It is seen from the above that the relay 3! will be operated to connect the sound reproducing device only when the signal source includes a signal of the proper frequency to actuate contactor 39 and a tone of the frequency to which the filter of the unit to is tuned to provide energy for application to the selective responsive device? In circuits actually constructed, electromechanical contactors wereused. which are.- responsivetofrequencies in the rangezof 200to. 400 cyclesand the'filter in the: unit 50. was tunedto select'tonesin afrequency of around 3000. cyc1es. As; the twocalling frequencies must: be simultaneously received by the selective calling sys::- tem: before operation. of the. system, the. likelihood of accidental. actuation of the system by noise or voice signals intended for" other stations is very unlikely. By. use of contactors having different frequencies and by tuning the filters to different frequencies it is; apparent. that the system can be? made to respondto. a: great; hum.- be'rof calls, For example, satisfactory operation was obtained whenusing' reedsresponsive'to ten different? frequencies for the; contactors. and. by

providing filters tuned. to ten. difierent: tone signals; As'the. reed frequencies and tone signals are indifferent: frequency bands, the use of ten spectively, and are: connected in. series with. re.- sistor; 'l-T andcondenser l8v for: charging the condenserv 18 from the voltage applied tov terminal 19 by the unit 14-. A resistor 86. is bridged across the condenser 18 to tend to discharge the same in. the. same manner as. described. with respect to Fig-.3. By providing contactors l1; and 12 which are-responsive to different frequencies and by applying signals'including these frequencies to the windings of the contactors, it is. apparent that the-intermittent and alternate action; of the contactors will result. The contactors will. besimul- ,taneously closed only occasionally, the? frequency being. equal. to thebeat frequency between the two frequencies to whichthe contactors: are'responsive.

The; selective. responsive device H3: is. in accordance: with the-'copending application of Ralph J.

Lense and Alfred E; Holzinger, Serial No. 749,952,

filed. May 2-3,. 1947, and accordingly will. not. be described indetail. However; a general description. of theoperation of this device. follows.

Whenthe contactor TI. is closed and contactor 1.2 isopen,theicondensers lli.-and;l8v will become charged by thevoltage across. terminalm. Now I when the" contactor i2;- is closed and contactor H open, the. condenser Hi is shorted tothereby removethe charge from. this condenser: and con.- ..d'ensersl5.- and (8- are. connected acrossithe voltage provided by the unit 1-4. Thev selective" re:- sponsive device is Will, of course, only operate when'a voltage is applied: thereto by unit. 14 and this depends upon; application of a particular tone. thereto from the transformer. 73. in the same manner asdescribed' with reference'to. rectifier 59. Continuous'actionin this'mannerwill cause increasing increments-of charge tor-be produced on condenser. F8 until the voltage. across this condenser approaches the voltage across termina s l9. This'voltage'is applied to' the. grid. 8| oftriode. 82 and when. this. voltage; reaches .a' predetermined valuesufiicient; output current.- will flow" in; the tube. to actuate;- the relay 833 causing" the-contacts 84 thereof tobe. operated-Lin a predetermined manner. It is; obvious. that. these contacts can be used in the. manner. illustrated inFig. 3. to connectza sound reproducing: device to; one of the. secondary windings of the trans:- former13.

From the above. description, it: can:v be clearly seen that for operation of the: relay 83 themcoming, signalmustinclude the frequenciesarequiredtoactuate contactors'll and l"2;as-well.as thetone to Whichthei filter of. the unit 14 isituned inorder to provide a voltage for the-.selectiveresponsive device iii. Although. the. calling; signals are transmitted over. the samechannel-aszvoice communications, it is. apparent. that the likelihood. thatthe. voice signals will include. therthree frequencies requiredfor actuating the device 10 is. very remote. Similarly the chance. of noise including these frequenciesisvery remote. Itis pointed out here that occasional. voice. or noise signal of the frequency to whichthe. filter isrresponsive may be present and random. signals. of the frequencies required for operation of the contactors 72 may also be present withoutcausing operation of the selective. calling system. Although such random. signals. will charge the condenser '48 somewhat, the resistor v is. at-all times tending todischarge. thev condenser. 1B and thereby prevents a charge accumulating thereon due to charge increments receivedover. a. relatively long time interval. The requirement. for

three separate signals for actuating the. selective responsive device also increases the. number of distinct calls whichcan be used for. thesystem. The use of contactors available in ten different frequencies and being used twoat a time as illustrated in Fig. 3, combined with ten different energizing tones provides a total of 450 different calls. This system, therefore, is applicable to a communications network having a large. number of stations.

It is obvious that the generalsystem illustrated in Fig; 1 is adaptable to a system. using either amplitude, angular, or pulse modulation, thecalling signals being'vvithin" the audio frequency range and being transmitted over this same channel with the voice signals. However, as thesignals to which the reeds are responsive are of relatively low'f'requency, these frequencies would not provide sufiicient deviation of the carrier for efficient' transmission in the case of a frequency modulated transmitter employing a phase modulator and equalized. This defect'can be overcome by modulating the tone frequency which in .the vicinity of'3000 cycles with the signal 011' signals for-actuating the contactors to thereby" provide sufficient energy" for driving the modulator; Re;- ferri'ng to Fig; 1 the mixer would be a modulator for modulating the frequency from one of' the tone generators" (the higher frequency) by the frequency of the other tone generator;

Fig; 5 illustrates a selective calling system adapted to respond to a composite'signa]. including a tone in' the audio frequency range which serves as a carrier and. is modulated by one or more signals of lower frequency: The: transformer' may be connected to any signalsource such as the output transformer ofthe audio amplifier 24 of Fig. 1' andinclud'es a pair'ofsecondary windings 9! and 92. The'windi'ng 9i is'conn'ected toa' signal reproducing device 93 and the winding 92 is connected to a selective -calling'systema The selective calling system includes a bandipassfilt'er .94.; adapted to? pass: the carrier tone: the sidetype of modulation used in the transmitter.

loading of the line.

bands which include the signals for actuating the reeds. The tone used as a carrier may be used for supplying the energy for the system and a plurality of systems can be selectively energized by providing tones of different frequencies sufiiciently separated so that a filter such as a standard reactance type filter will eliminate all of the tones (carriers) except the one which it was intended to pass. The composite signal is then amplified by amplifier 95 and applied to detector 9% and rectifier 91. The modulating signals are derived by the detector 96 and applied to the signal responsive device 98. The rectifier 91 can be any suitable rectifier and should include a filter for smoothing the'output before application to the selective responsive device 98.

' The system of Fig. may be used with either an amplitude modulated system or an angular 'modulated system. The unit 9'6 will be either an amplitude modulation detector or a discrimi nator for angular modulation depending upon the If a frequency modulation system is used the filter 94 may be provided between the amplifier 95 and the rectifier 91 as the discriminator will be sensitive only to a particular carrier frequency and elimination of other frequencies is not required. By placing the filter 94 in this position the band which the filter must pass may be narrowed as the filter will only need to pass the carrier and will not need to pass the side bands which include the signals for actuating the selective responsive device 98.

Theselective responsive device of Fig. 5 may be I; of the type shown at 46 in Fig. 3 which is responsive to a single calling signal or the type shown at in Fig. 4 which requires two separate signals for actuating the same. As previously stated, by using a single contactor as illustrated by the selective responsive device in Fig. 3 it is possible to use 100 different calls and by using two diiier- .ent reeds as in the system of Fig. 4, it is possible to obtain 450 different calls. The selective responsive device includes contacts 99 for connecting the sound reproducing device 93 to the secondary winding 9| in the manner previously described.

In Fig. 6 there is illustrated the detailed circuit of a selective calling system of the type generally shown in Fig. 5 which is adapted to be operated by a tone frequency modulated by two signal frequencies. In Fig. 6 the transformer I00 may be connected to any signal source such as the audio output of a radio receiver. The composite signal is fed from the transformer to a buffer amplifier l0I which selects the desired energizing tone,-in-

creases the signal strength thereof, and prevents The amplified and selected signal is applied through blocking condenser I02 to discriminator I 03 wherein the modulating frequencies are derived from the composite signal. Although various types of discriminators can be used the circuit illustrated utilizes a discriminator which is generally of the type disclosed in .Patent No. 2,404,359. The signal frequencies are nal responsive device I0 of Fig.3. The signal from the amplifier IOI is also applied through condenser I05 to a rectifier unit I05 which provides a direct current voltage for operation of the selec- -tive responsive device I04. k The amplifier I 0] includesa trio'de .vacuum tube I0! and an output circuit I08 which is tuned to the frequency of the energizing tone. This output circuit is coupled to a second tuned circuit I09 which is connected through condensers I02 and I05 to the discriminator I03 and the rectifier I06 respectively. The double tuned circuit I08 and I09 must provide suflicient band width to pass the energizing tone which functions as a carrier, as well as the side bands which contain the tones for actuating the reeds of the selective responsive device I04 as will be described in detail. I

Referring now more particularly to the operation of discriminator I03, the signal is applied from the amplifier to a pair of rectifiying paths, one including condenser H0 and rectifier III which is bridged by resistor H2 and condenser H3 and the second including condenser H4 and rectifier H5 which is bridged by resistor Ilt'and condenser II'I. An inductor H8 is bridged across the condensers H0 and H4 to tune the input circuit approximately to the frequency of the tone to be received. The condensers H3 and H4 are of slightly different value to unbalance the two bridges of the discriminator as fully described in the patent referred to above. Inductor H9 and condenser I2I are of such values to be series resonant at the energizing tone frequency. Resistor I20 lowers the Q of the series resonant circuit, thus providing the necessary by-pass bandwidth. Owing to the presence of the two above mentioned circuits, the discriminator operates only on the particular carrier signal to which it is tuned and rejects all others. As is well known, the discriminator is not affected by amplitude variations in the tone and, therefore, interference in the form of amplitude variations will be rejected and will not be applied to the selective responsive device. The modulating signals are derived from the tone by the discriminator and are applied through blocking condenser I22 to a power amplifier tube I23 after which they are applied to the windings of the contactors I24 and I25 of the selective responsive device I04. The detailed circuit of the selective responsive device I04 will not be described as this is identical to the selective responsive device I0 of Fig. 4 previously described.

The rectifier unit I06 includes a rectifier I30 and an integrator circuit comprising a resistor I3I and a condenser I32 for smoothing the rectifier output and providing a steady voltage for the selective responsive device. As the amplifier IOI is selective, the rectifier I06 will cause energization of the selective responsive device I04 only when a particular tone is received. As

previously stated, the discriminator I03 will not provide output signals for the contactors unless the particular tone to which the discriminator is tuned is received as a carrier, and further the contactors will not be actuated unless signals of the particular frequencies to which the contactors respond are applied thereto from the di'scriminator. The selective energization of the selective responsive device adds further selectivity to the system. Therefore, it is obvious that the .isatisfactory operation may be obtained without 9 the use of a rectifier which is effective to-energize the .selective responsive device only when a .particularwenergizing tone is applied thereto. Therefore, the rectifier I06 mayrbe replaced by any suitablesource of direct current and satisfactory operation of the system will still be obtained. It is to be pointed out, however, that added selectivity is obtained by providing the rectifier as shown in Fig. 6 for selectively energizing the selective responsive device.

As previously stated, examples of filter and rectifier units suitable for use in the systems of Figs. Blend 4 are shown in Figs. '7 and 8. Fig. '7 illustrates a simple tuned filter including a primary winding I40 tuned by condenser MI and connected in parallel therewith across the winding I39. Resistor I38 is provided to prevent detuning of the circuit. The winding I39 provides audio and calling signals and may be connected in a-system as the winding 34 of Fig. 3. The primary winding 1-46 is inductively coupled to a secondary winding I42 which is tuned by condenser I43. The output of the filter is applied to arectifier I 24 which provides a direct current which is smoothed out by a filter comprising resistor I45 and condenser I46. This output is suitable for energizing a selective .responsive device such as '46 .of Fig. 3 and may be applied to the terminals 43 thereof.

In :Fig. 8 there is disclosed a selective amplifier which selects and amplifies the energizing tone and then applies it to a rectifier which provides direct current for the selective responsive device. The amplifier isconnected to a source of signals which may be a Winding I corresponding to the secondary winding 36 in Fig.3. The amplifier includes a triode I5I and a filter which is tuned so that it passes a tone of a particular frequency only. The triode includes a grid I52 which is connected tothe winding I56 by blocking condenser I53 and a series resistor I56 and is biased by resistor I55. The filter is connected both "to the grid I52 and the plate I56 of the triode and includes a high pass branch including series condensers I5 and I58 and shunt resistor I59 and a low pass branch including series resistors I69 and MI and shunt condenser I62. A blocking condenser I63 is provided in series with the resistors I66 and I6I to prevent application of signals from the winding I56 to the plate I56 of the triode. 'It is pointed out that other well known types of filters can be used, the particular filter illustrated being merely by way of example. The plate I56 of the triode is connected through resistor I-64 to a source of +13 voltage and through blocking condenser I65 to rectifier I66. The rectifier I66 .is connected through an integrator network comprising resistor l6? and condenser I66 which provides a direct current voltage which is substantially free of fluctuations. This voltage may be used to energize a selective responsive device and, for example, may be applied to terminals 43 in Fig. 3 or terminals Is in Fig. 4.

The various selective responsive systems disclosed above may be provided by relatively simple and inexpensive components and the resulting equipment is rugged and comparatively small as is'required for mobile use. Equipment in accordance with the invention has been found to be very reliable and highly selective in use. The simultaneous occurrence of an energizing tone of one frequency and one or more tones of lower frequencies for energizing reeds as required for operation of the calling system will seldom be present in noise, voiceorfmusic signals to cause unintentional actuation of the calling s stem. Also, .thearrangement wherein the relatively'low frequency reed signals are used to modulate the higher frequency tone signals makes a system which is particularly-suitable for use in frequency .1. A selective control-system including in com bination, first frequency selectivemeans responsive to a signal of a first frequency, said first frequency selective means including means producing a directcur-rent output voltagein response to the application of said first signal thereto, second frequency selective means including means providing an intermittently conducting path in response to the application thereto of a secondsignal including at least one different frequency, means for simultaneously applying signals to said first and second frequency selective means, condenser means, and circuit means including said second frequency selective means for connecting said condenser 'means to said first frequency selective means for charging said condenser means from said direct current output voltage :through said intermittently conducting path, said circuit means providing charging current for said condenser means only in response to the simultaneousapplication to said system .of said first and second signals. U

2. .A selectiveoontrol system in accordance with claim 1, in which said first frequency selective means includes filter means for selecting said first signal-and rectifier means connected to said filter means for producing a direct current voltage from said first signal, and said second frequency selective means includes a vibratory reed having anatur'al frequency the same as said different frequency of said secondsignal, means for excitingsaid reed by said secondsignal, and contactmeans on said reed which are intermittently closed as said reed vibrates.

3. A selective control system in accordance with claim 1, in which said first frequencyselective means includes filter means for selecting said first signal and rectifier means connected to said filter means for producing-a direct current voltage from said first signal, and said second frequency selective means includes :a pair of vibratory reeds having natural frequencies the same as two frequencies of said second signal which are different from said first frequency, means for exciting said reeds by said second signal, contact means on said reeds which are intermittently closed as said reeds vibrate, and means connecting said contact means in series.

4. A selective control system including in combination, electromechanical means including contact means which are closed in response to a signal of a first frequency, frequency selective means for passing a signal of a second frequency only, means for simultaneously applying signals to said electromechanical means and said fre= quency selective means, rectifier means, com denser means, circuit means connecting said rec tifier means to said frequency selective means so that said rectifier means produces a direct current output voltage in response to the application of a signal of said second frequency to said frequency selective means, and second circuit means including said contact means of said electr'omechanical means for selectively connecting said condenser means to said rectifier means in response to the application of a signal of said first frequency to said electromechanical means, for charging said condenser means from said direct current voltage produced by said rectifier means through said contact means of said electromechanical means, whereby said condenser means is substantially charged to produce a control voltage only in response to the simultaneous application to said system of signals of said first and second frequencies.

5. A selective control system in accordance with claim 4, in which said electromechanical means is responsive to a signal of a relatively low frequency and said frequency selective means is responsive to a signal of a relatively high frequency.

6. A selective control system including in combination, a first and second electromechanical means individually responsive to signals of first and second frequencies, frequency selective means for passing a signal of a third frequency only, means for simultaneously applying signals to said electromechanical means and said frequency selective means, rectifier means, condenser means, circuit means connecting said rectifier means to said ferquency selective means so that said rectifier means produces a direct current output voltage in response to the application of a signal of said second frequency to said frequency selective means, each of said electromechanical means including means rendered conducting when a signal to which the particular means is responsive is applied thereto and second circuit means including said first and second electromechanical means for selectively connecting said condenser means to said rectifier means in response to the application of signals of said first and second frequencies to said electromechanical means, for charging said condenser means from said direct current voltage produced by said rectifier means, whereby said condenser means is charged to produce a predetermined control voltage only in response to the simultaneous application to said system of signals of said first and second frequencies for a predetermined time interval.

7. A selective system responsive to a composite signal including a first signal of a first frequency which is modulated by a second signal including a second frequency differing from said first frequency, said system including in combination, first frequency selective means including means producing a direct current output voltage in responsev to the application of a signal of said first frequency thereto, detector means for deriving the modulating signal from a modulated signal, second frequency selective means including means providing an intermittently conducting path in response to the application thereto of a signal including said second frequency, means connecting said detector means to said second frequency selective means for applying the derived modulating signal thereto, condenser means, and circuit means including said second frequency selective means for connecting said condenser means to said first frequency selective means for charging said condenser means from said direct current output voltage through said intermittently conducting path, said circuit means providing charging current for said condenser means only in r-' sponse to the application to said system of a composite signal simultaneously including said first signal modulated by said second signal.

8. A selective system in accordance with claim 7, in which said first frequency selective means includes filter means which passes only a signal of a first relatively high frequency and rectifier means for producing a direct current from the signal passed by said filter, and said second frequency selective means includes electromechanical means having contacts which intermittently close in response to the application thereto of a second relatively low frequency.

9. A selective system responsive to a composite signal including a first signal of a first frequency which is frequency modulated by a second signal including two other frequencies, said system including in combination, first frequency selective means responsive to said first frequency for passing said first signal, rectifier means connected to said first frequency selective means for producing a direct current output voltage from the signal passed thereby, detector means connected to said first frequency selective means for deriving the modulating signal from a modulated signal applied thereto, second frequency selective means connected to said detector including a pair of electromechanical contactors each providing an intermittently conducting path in response to the application thereto of one of said other frequencies, condenser means, and circuit means connecting said condenser means and said electromechanical contactors in series to said rectifier means for charging said condenser means from said direct current output voltage of said rectifier means through said intermittently conducting paths of said electromechanical contactors, said circuit means providing charging current for said condenser means only in response to the application to said system of a composite signal including a first signal of said first frequency which is frequency modulated by a second signal including said two other frequencies.

DANIEL E. NOBLE.

REFERENCES CITED The following references are of record in the file of this patent:

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